BigCommerce Theme Update & Syntax Check

I need an expert to upgrade BigCommerce theme to a new version 2.7.4 and ensure all syntax is correct.
https://www.bigcommerce.com.au/theme/covent-garden-contemporary/

- No specific deadline: I'm flexible with the timing.
- Moderate customizations: Expect some layout changes and custom pages.
- Concerns:
- Functionality and features: Make sure all existing functionalities remain intact and fully operational.
- Visual design and layout: Ensure the aesthetic integrity of the site is maintained post-update.
- Speed and performance: Optimization in these areas post-update is crucial.

The ideal candidate would have:
- Extensive experience in BigCommerce theme updates.
- Strong understanding of BigCommerce syntax.
- A keen eye for detail to ensure all custom features and designs are properly transitioned.
- Experience in optimizing site speed and performance.

The original theme version is 2.3.2

The attached image shows the customised pages in the theme.

CSS for layout will remain mostly the same.

Ensure that theme is using an updated SCSS framework.
https://developer.bigcommerce.com/docs/storefront/stencil/cli/incompatible-directives

Resolve issue re: undefined variables.

Allow for possible variations.